1. Field of the Invention
The invention pertains to the field of chemical analysis of chemical mechanical planarization (xe2x80x9cCMPxe2x80x9d) slurries. More specifically, it relates to an optical method of determining the chemical constituents of a CMP slurry.
2. Statement of the Problem
Integrated circuits are manufactured by depositing layers of various materials and then patterning the layers using mask and etch processes. After patterning, the incomplete integrated circuit surface usually is irregular, having steps, wells, and other structures. When a fresh layer of material is deposited over this irregular structure, the deposition generally follows the irregularities, resulting in an irregular surface for the fresh layer. Usually this is undesirable, and it is necessary to smooth the surface. CMP processes are used to planarize and smooth such surfaces. CMP has emerged as the preferred method of planarization for manufacture of multiple layer semiconductor wafers having feature sizes less than or equal to 0.35 microns. CMP is also used in the optics industry for similar purposes.
CMP process slurries typically consist of a high concentration of sub-micron size abrasive particles, such SiO2, Al2O3 or CeO2 particles, suspended in a chemically active agent, such as an acid or base solution. The abrasive concentration is generally 4% to 18% solids by weight. SiO2 slurries are referred to in the art as xe2x80x9coxidexe2x80x9d slurries, and Al2O3 slurries are referred to as xe2x80x9cmetalxe2x80x9d slurries. CMP slurry manufacturers attempt to produce slurries that consist predominantly of particles less than 1.0 or even 0.5 microns in size.
Since CMP slurries intimately contact the materials out of which the integrated circuit or optics components are made, it is important that the chemical content of the slurry be known and controlled. For example, materials that are planarized and chemicals used in other integrated circuit manufacturing processes can get into the slurry and affect subsequent layers when they are planarized. Further, the chemicals and materials that make up the slurry can decompose over time. Therefore, it is important to be able to periodically check the chemical composition of a CMP slurry. In the past, this has been done by analytical chemical means, such as titration. However, such methods are slow, consume expensive chemicals, and create a hazardous chemical waste stream. Automated titration systems may require thirty minutes to make a measurement, whereas optical methods typically require only one to ten seconds.
Recently, it has been suggested that an optical device can be used to determine some chemicals in a CMP slurry. See U.S. Pat. No. 6,267,641 B1 issued Jul. 31, 2001 to J. F. Vanell and C. B. Bray. This patent describe a means of measuring the hydrogen peroxide content of CMP slurries, by inserting a commercially available refractometer into a sample chamber containing the slurry. However, a refractometer relies on an index of refractive difference between hydrogen peroxide and water, and is not very sensitive, due to the substantially opaque nature of the slurry. The minimum detectable concentration of hydrogen peroxide, using a refractometer, is on the order of 0.5%. See A. Misra and M. L. Fisher: xe2x80x9cA Robust On-Line Technique for Determination of Oxidizer Concentration in CMP Slurriesxe2x80x9d, SEMICON West 2001, Symposium Proceedings for Innovations in Semiconductor Manufacturing; and Photonics Handbook, Book 4, 44th Edition, 1998, Laurin Publishing. Those CMP slurries which utilize hydrogen peroxide as the active chemical have a concentration of 2%-5% hydrogen peroxide by weight. For these relatively high concentrations of active chemicals, refractometers yield an adequate solution. However, for other chemicals, much more sensitivity is required. For example, organic liquids may be used as the chemically active agent, in concentrations down to 200 ppmV (0.02%), far below the detectable limit exhibited by refractometers for measurement of hydrogen peroxide. Furthermore, refractive index measurements are very temperature sensitive, and require tight control of the sample temperature to obtain accurate results.
Thus, there remains a need for a real-time probe to enable determination of the chemical content of a CMP slurry that does not require removal and analysis of a portion of the CMP slurry.
It is, accordingly, one object of the invention to provide a probe and/or system which provides real-time measurement of CMP slurry particle size distributions and/or change of the particle size distribution. Another object of the invention is to provide a quality control process to detect acceptable and unacceptable CMP slurries, real-time, in a manufacturing environment. Yet another object of the invention is to provide systems and methods for detecting CMP slurry particle size distributions and/or changes in such distributions. These and other objects of the invention are apparent within the description which follows.
The present invention overcomes the problems outlined above, and advances the art, by providing real-time systems, methods and/or probes for continuous chemical analysis of undiluted CMP slurry. The CMP slurry can include a broad range of chemicals. The systems and methods of the invention provide high sensitivity to small changes in the chemical characteristics of the CMP slurry, and preferably with autonomous operation in an industrial environment. These advantages are obtained, in certain aspects, by measuring spectral transmission through undiluted CMP slurry samples (typically through a slurry xe2x80x9cflowxe2x80x9d). The spectral transmission measurements are made at one or more wavelengths, and preferably at three or more wavelengths. In particular, the preferred system of the invention provides an apparatus and a method for accurately determining the concentration of active chemicals in CMP slurry, through on-line, continuous sampling of undiluted slurry, using spectroscopic means.
In one aspect, a probe according to the invention measures the concentration of active chemical species in CMP slurry with undiluted, continuous, on-line sampling for real-time process control, the probe preferably including a plurality of light sources, a detector system which includes one or more fixed grating linear detector array spectrometers and sample cells, a three-position chopper, and an optical pathway for transmitting light from the light sources through the sample cells and then to the detector system or spectrometers. A computer or microprocessor receives detector signals and performs calculations to determine the concentration of chemical species. The sample cells are specially constructed to reduce optical depth in the slurry, which permits chemical concentration measurements without dilution of the slurry.
In another aspect, the invention preferably determines a slope of transmission as a function of the wavelengths. This determination of a change in the slope is preferably made xe2x80x9cover timexe2x80x9d such that a change in slope indicates a change in the chemical content of the slurry. Further, the slope is preferably determined logarithmically. That is, the process preferably includes the step of determining a logarithmic slope of transmission as a function of the wavelengths.
In another aspect, the invention provides a method and apparatus for determining a change in the logarithmic slope over time. The change in logarithmic slope indicates a change in chemical concentration.
In one aspect of the invention, radiation wavelengths selected for transmission through the CMP slurry are isolated by a grating or other dispersive optical element (e.g., a prism). The wavelengths can alternatively be determined by using a laser with a known wavelength emission. In a preferred aspect, wavelength selection is made through use of one or more bandpass filters (and preferably three filters), such as within a filter wheel. A combination of the above spectral discriminators can also be used, as needed, in accord with the invention. An order sorting filter is optionally included with any of these dispersive discriminators.
In yet another aspect, the invention provides a method and apparatus that compares the transmission to a reference transmission indicative of a preferred chemical content within a flow of the CMP slurry. Preferably, a method and apparatus for storing the reference transmission in memory is provided and a comparison between the reference transmission and the actual transmission is made electronically and in real time.
Alternatively, the invention provides a method of (a) storing a plurality of reference transmissions, where each reference transmission corresponds to a particular CMP slurry flow and chemical content, and (b) selecting one reference transmission and comparing the transmission to the selected reference transmission.
A process of the invention can further include comparing transmission information with an empirical curve of extinction efficiency versus particle size diameter to determine chemical content of the slurry.
The invention also provides a system for evaluating CMP slurry quality in a process. In this aspect, a light source generates a beam of electromagnetic radiation for transmission through a flow of the slurry. A spectral discriminator isolates at least two, and preferably three, wavelength bands of the radiation prior to transmission of the radiation through the flow. A detector detects radiation transmitted through the flow. A processor evaluates transmission of the wavelength bands through the flow to determine chemical changes in the CMP slurry.
In one aspect, a computer with a processor serves as the processor of the system, to process signals and to make determinations and calculations. Those skilled in the art should appreciate that other processors, e.g., an ASIC, can alternatively be used.
In yet another aspect, a system of the invention includes memory, coupled to the processor, to store one or more reference transmissions. Each of the reference transmissions corresponds to a particular CMP slurry flow and chemical make up. The processor selects one reference transmission and compares the transmission through the flow to the selected reference transmission to detect changes in the particle size distribution. The memory can further store other reference data for comparison to other changes in chemical characteristics of the CMP slurry, in accord with the invention.
In still another aspect, the memory of the invention can store data indicative of extinction efficiency as a function of chemical content. The processor then compares the transmission to the data to determine chemical content of the slurry.
The invention is next described further in connection with preferred embodiments, and it will become apparent that various additions, subtractions, and modifications can be made by those skilled in the art without departing from the scope of the invention.